Alginate bead microreactors integrated with SDS-MIL-88A and Cu-biochar for adsorption-coupled Fenton-like degradation of tetracycline at neutral pH

Abstract

Herein, this study provides a solution to the shortcomings of most heterogeneous Fenton/Fenton-like catalysts, such as high dispersibility and difficulty of separation from the reaction medium. The activity of the Fenton MIL-88A catalyst was enhanced by the functionality of sodium dodecyl sulfate (SDS), and the efficiency of the Fenton-like biochar catalyst was improved by copper doping. The highly efficacious SDS-MIL88A/Cu-BC catalyst was impregnated into the sodium alginate (SA) and shaped into a bead form, yielding the SDS-MIL88A/Cu-BC@SA beads. Various characterization instruments confirmed the successful synthesis of the SDS-MIL88A/Cu-BC@SA beads. The SDS-MIL88A/Cu-BC@SA was applied in the Fenton-like decomposition of the antibiotic tetracycline (Tc) drug, and the optimal reaction conditions were scrutinized after sequential catalytic experiments. The SDS-MIL88A/Cu-BC@SA beads achieved 95.12% Tc decomposition within 60 min, with 44.91% adsorption at pH = 7, dose of the beads = 20 mg, Tc concentration = 50 mg L⁻¹, system temperature = 20 °C, and H₂O₂ = 100 mg L⁻¹. In addition, TOC analysis showed approximately 65.9% TOC removal, confirming the partial mineralization of Tc molecules and supporting the formation of smaller degradation intermediates during the adsorption-coupled Fenton-like process. The kinetic investigations of the Fenton-like decomposition of Tc by SDS-MIL88A/Cu-BC@SA obeyed the Second-order model. The proposed mechanistic study suggested that the adsorption of Tc occurs via coordination bonds, hydrogen bonding, n-pi interactions, pi–pi interactions, and Lewis acid–base interactions. Furthermore, the Fenton-like degradation of Tc occurred through the activation of H₂O₂ by ferrous, monovalent copper ions and EPFRs, with the recovery of the active metal species by SDS and EPFRs. The recyclability examination of the SDS-MIL88A/Cu-BC@SA beads revealed that the decomposition % of Tc was over 85% after the fifth catalytic run.

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1 Introduction

Antibiotics are anti-inflammatory and anti-apoptotic and are used immensely in curing many diseases, especially the recent pandemic, rendering them a substantial reason for water contamination.^1,2 The residual antibiotics enter water bodies from various sources, including animal farming, pharmaceutical manufacturers, and hospitals.³ Unfortunately, when these antibiotic residuals exist in water bodies, they raise the resistance of bacteria against the antibiotics. Among the widespread antibiotics in wastewater is tetracycline (Tc), which is applied to cure infections in the urinary tract, stomach, respiratory tract, and skin. Humans can only digest around fifty percent of the Tc doses, while the rest is ejected in urine.⁴ So, treatment strategies are developed day-by-day for remedying wastewater from antibiotic residuals like Tc, comprising adsorption, membranes, catalysis, etc.⁵

Recently, advanced oxidation processes have drawn considerations in water remediation because these processes not only remove contaminants but also degrade them into less toxic or un-toxic compounds.^6–8 Fenton reaction is a hydroxyl-based oxidation process that proceeds by activating the hydrogen peroxide (oxidant) with an iron-containing catalyst to yield the hydroxyl radical (˙OH) that degrades the contaminants.⁹ Notably, advanced studies have concluded the possibility of applying a catalyst that does not include iron in activating H₂O₂, known as a Fenton-like catalyst, which may be metallic or non-metallic.¹⁰ Fenton and Fenton-like reactions provide promising remediation efficacy towards antibiotics like Tc in a short reaction time with acceptable cost.

Iron-based metal–organic frameworks, represented in a MIL family, have exhibited outstanding Fenton degradation performance towards many antibiotics.^11,12 MIL88A has been deemed the best candidate as a Fenton catalyst because of its chemical stability during the reaction and its stability in the aquatic media.¹³ The eco-friendly advantage of MIL88A distinguishes it from the other MIL members since it can be produced using water as a solvent instead of the harmful dimethyl formamide.¹¹ In addition, the flexibility of MIL88A facilitates the amelioration of its catalytic activity by functionalization with active groups. Nevertheless, the high dispersity of MIL88A limits its applicability since its collection from the catalytic medium is a challenging process, increasing the possibility of yielding secondary contaminants and diminishing its recyclability. Therefore, pioneering assessments have concluded the viability of the impregnation of MIL88A into shaped beads from polymeric materials and decoration it with magnetic substances to outdo its hard separation from catalytic media.^14–16

Biochar, BC is a promising non-metallic Fenton-like catalyst that has demonstrated excellent activity towards the degradation of antibiotics since it contains the environmental persistent free radicals (EPFRs) that can share with electrons for the production of ˙OH.¹⁷ Additionally, BC owns other remarkable characteristics that enable it to be an eminent Fenton-like catalyst, such as large surface area, porous topology, mechanical strength, chemical stability, and cost-effectiveness.¹⁸ The easy functionality of BC boosts its catalytic activity throughout doping with active transition metals (iron, strontium, tin, and copper).¹⁹ Generally, BC is yielded by pyrolyzing a feedstock in an environment free of oxygen.²⁰ This feedstock may be a waste from crops, agriculture, forestry, poultry manure, algae, and sewage sludge.²¹ Impressively, the production of BC provides an efficient catalyst and contributes to recycling the wastes, outdoing their negative impacts on the environment.²² Despite the advantages of BC, its tiny particle size and high dispersity in aquatic media make its separation after the catalytic reaction a big challenge, in which the traditional ways may consume a long time without a perfect result.

Sodium alginate (SA) is a polysaccharide polymer extracted from seaweeds, in particular brown varieties such as Ascophyllum nodosum, Laminaria digitata, Laminaria japonica, and Macrocystis pyrifera.²³ The individual merits of SA render it versatile in many sectors, where SA enjoys biocompatibility, non-toxicity, reusability, abundant hydroxyl and carboxyl groups, chemical stability, mild gelation, and inexpensive production cost.^24–26 Furthermore, the easy molding feature of SA enables its shaping in different forms, such as bead, hydrogel, membrane, and sphere. Thanks to this feature, SA can overcome the high dispersity and difficult separation of many substances by incorporating them within the SA matrix. For this purpose, recent studies have underscored the remarkable efficiency of the SA beads to be a supporter of heterogeneous catalysts, revealing a minimized loss of mass and amelioration in the reusability of the catalyst.^27,28

Our study attempts to overcome a recyclable Fenton-like catalyst with high catalytic activity toward the antibiotic Tc. Firstly, the catalytic activity of MIL-88A was enhanced via the functionality with sodium dodecyl sulfate (SDS) to endow it with excess active sites. Then, SDS-MIL88A was blended with the BC to exploit its EPFRs that boost the ˙OH production in the catalytic system. Additionally, the catalytic efficacy of BC was improved by copper doping. Finally, SDS-MIL88A/Cu-BC was impregnated into SA beads to overcome the high-dispersity of MIL88A and BC and improve the recyclability of the SDS-MIL88A/Cu-BC@SA beads. The formation and characteristics of the SDS-MIL88A/Cu-BC@SA beads were examined using various characterization instruments. The optimal catalytic conditions for the Tc/SDS-MIL88A/Cu-BC@SA system were determined by analyzing different catalytic parameters in an adsorption/Fenton-like mode. The Fenton-like degradation data were analyzed using first-order and second-order kinetic models. The possible mechanism of the Tc adsorption/Fenton-like degradation by SDS-MIL88A/Cu-BC@SA beads was investigated, based on the X-ray photoelectron spectroscopy. The reusability of the SDS-MIL88A/Cu-BC@SA beads was scrutinized for five adsorption/Fenton-like runs of Tc.

2 Experimental section

2.1 Fabrication of SDS-MIL88A

SDS-MIL88A was synthesized throughout the hydrothermal approach, as elucidated in the following procedure. In a beaker, 2.7 g of ferric chloride was soaked in 65 mL of double-distilled water, followed by adding 1.16 g of fumaric acid to the orange ferric solution. The ferric/fumaric acid solution was stirred for 30 minutes, after which 2.88 g of SDS was added, maintaining the solution under stirring for two hours. Next, the resultant bright yellow solution was transferred into an autoclave and heated for 22 hours at 85 °C. Finally, the SDS-MIL88A(Fe) powder was collected, rinsed with double-distilled water, and heated in an oven at 85 °C to dry. The pure MIL88A(Fe) was fabricated using the same method, but without the SDS addition step.

2.2 Fabrication of Cu-BC

Coffee waste was the biomass for fabricating BC, collecting it from a coffee shop in Alexandria, Egypt. The coffee waste was dried at 50 °C in an oven until completely dry. Then, the coffee waste was calcinated in an environment with a limited oxygen for 3 hours in a muffle furnace at 500 °C, resulting in a deep black solid of BC. Subsequently, 1.0 g of the coffee waste-derived BC was suspended in 35 mL of double-distilled water and stirred potently until a complete dispersion. 2.0 g of CuCl₂·2H₂O was soaked in the BC suspension with stirring the Cu/BC mixture for 30 minutes. The liquid in the Cu-BC suspension was evaporated at 100 °C in an oven for 12 hours, then the Cu-BC was pyrolyzed by a muffle furnace for 3 hours at 500 °C.

2.3 Fabrication of SDS-MIL88A/Cu-BC@SA beads

The SDS-MIL88A/Cu-BC@SA beads were prepared using the following steps. First, dissolve 0.5 g of SA in 10 mL of double-distilled water with stirring until a clear gel is obtained. Next, suspend 0.25 g of SDS-MIL88A and 0.25 g of Cu-BC in the SA gel, continuing to stir for 15 minutes. Then, drop the SDS-MIL88A/Cu-BC/SA gel into a CaCl₂ solution (2 %wt/v) using a syringe, allowing the beads to cure for 30 minutes. Finally, detach the SDS-MIL88A/Cu-BC@SA beads using a colander, rinse, and store them in double-distilled water. Scheme 1 represents the preparation steps of SDS-MIL88A/Cu-BC@SA beads.

Scheme 1 Preparation of SDS-MIL88A/Cu-BC@SA beads.

2.4 Batch degradation experiments

To determine the best catalytic conditions of the Tc/SDS-MIL88A/Cu-BC@SA system, several batch experiments were conducted as follow: (1) a comparison between the adsorption/catalytic activities between MIL88A, SDS-MIL88A, BC, Cu-BC, neat SA beads, and SDS-MIL88A/Cu-BC@SA beads toward Tc to indicate the positive impact of the functionalization of MIL88A and BC, as well as, reveal the synergetic effect between SDS-MIL88A, Cu-BC, and SA. (2) The selected pH of the Tc/SDS-MIL88A/Cu-BC@SA catalytic system was identified in light of the resultant degradation % of Tc at a scaled pH from 3 to 11. (3) The appropriate H₂O₂ concentration to effectively degrade Tc was demonstrated by proceeding the Tc Fenton-like reaction using different concentrations of H₂O₂ between 10 and 200 mg L⁻¹ (4) The degradation efficiency of Tc was studied using different weights of SDS-MIL88A/Cu-BC@SA, ranging from 7 to 25 mg. (5) The thermodynamics of the catalytic system was investigated from the experimental findings of the Tc Fenton-like reaction by SDS-MIL88A/Cu-BC@SA at raised temperatures from 20 to 50 °C. (6) The activity of SDS-MIL88A/Cu-BC@SA toward varied concentrations of Tc was evaluated by escalating the Tc concentrations from 50 to 300 mg L⁻¹ at constant catalytic parameters. Moreover, the durability of SDS-MIL88A/Cu-BC@SA was evaluated by reusing the beads for five Tc adsorption/Fenton-like cycles. Additionally, the governing reactive species was revealed by quenching the activity of SDS-MIL88A/Cu-BC@SA toward Tc using t-BuOH and TCM to scavenge ˙OH and O₂˙⁻, respectively. To determine the degradation efficiency, aliquots of the initial and treated Tc solutions were collected and analyzed using a spectrophotometer. The initial and final Tc concentrations were denoted as C₀ and C_t, respectively. The degradation efficiency was calculated using the following equation.

3 Results and discussion

3.1 Characterization of SDS-MIL88A/Cu-BC@SA

3.1.1 FT-IR. The successful fabrication of MIL88A, SDS-MIL88A, BC, Cu-BC, SA, and SDS-MIL88A/Cu-BC@SA beads was confirmed by investigating their chemical structures using FT-IR analysis, as illustrated in Fig. 1a. The FT-IR spectra of MIL88A, SDS-MIL88A, BC, Cu-BC, SA, and SDS-MIL88A/Cu-BC@SA beads showed a common broad peak in the whole spectra centered at around 3300 cm⁻¹, which is attributed to the hydroxyl group of the adsorbed H₂O. The FT-IR spectrum of MIL88A displays its corresponding band of Fe–O at a wavenumber of 574 cm⁻¹, implying the formation of the coordination Fe³⁺-fumaric acid bond. The absorption peaks of fumaric acid's carbonyl and hydroxyl groups emerged at 1707 and 3371 cm⁻¹, while its carboxyl peaks appeared at 1603 and 1396 cm⁻¹.^29,30 For the FT-IR spectrum of SDS-MIL88A, the associated absorption peaks to S–O of sulfate groups manifested at 1277 and 1052 cm⁻¹. Furthermore, the bending vibration of sp³ C–H is observed at 2856 cm⁻¹, and the stretching vibration is revealed at 2957 cm^−1.31,32 The appearance of the S–O and C–H of SDS in the SDS-MIL88A sample indicated the SDS functionality on the surface of MIL88A. The FT-IR spectrum of the coffee waste-derived BC showed the distinguishing peaks with carbonyl and carboxyl functional groups at 1509 and 1487 cm⁻¹. In addition, the absorption peaks at 1088 and 798 cm⁻¹ are ascribed to the C–O and C–H bonds.³³ For the FT-IR spectrum of Cu-BC, a noticeable shift of the carbonyl group to 1609 cm⁻¹ reflects its interaction with the copper species.³⁴ Additionally, a new absorption peak revealed at 450 cm⁻¹, which is ascribed to the Cu–O bond, suggesting the decoration of the BC surface with the Cu species. For the FT-IR spectrum of pure SA, the C–H vibrational bond of pyranose appeared at 796 cm⁻¹, and its stretching vibrational C–H bond revealed at 2916 cm⁻¹. The carbonyl peaks emerged at 1404 cm⁻¹ (asymmetric bond) and 1594 cm⁻¹ (symmetric bond).³⁵ The located absorption peak at 2333 cm⁻¹ is most likely due to the branched carboxyl group on the SA backbone.³⁶ The FT-IR spectrum of SDS-MIL88A/Cu-BC@SA beads showed the domination of the SA peaks due to the embedding of SDS-MIL88A and Cu-BC inside the polymeric shell of the SA beads.

Fig. 1 (a) FTIR, (b) XRD of MIL88A, SDS-MIL88A, BC, Cu-BC, SA, and SDS-MIL88A/Cu-BC@SA beads, and (c) zeta potential of SDS-MIL88A/Cu-BC@SA beads.

3.1.2 XRD. Fig. 1b displays the crystallographic patterns of MIL88A, SDS-MIL88A, BC, Cu-BC, SA, and SDS-MIL88A/Cu-BC@SA beads. The MIL88A pattern demonstrated its diffraction peaks at 2-theta of 10.28°, 11.12°, 12.99°, 15.30°, and 21.08°, which associates to 100, 101, 110, 012, and 022 planes.^37–39 For the SDS-MIL88A pattern, the belonging diffraction peaks to SDS emerged at 2-theta of 8.63°, 19.57°, 22.35°, 22.9°, 25.80°, 29.92°, and 31.83°, reflecting the successful SDS-functionalization on MIL88A.^40,41 The diffractogram of pure BC reveals the characteristic wide peak at 2-theta of 25°.⁴² For Cu-BC, the belonging diffraction peaks to CuO appeared at 31.70°, 32.32°, 39.68°, 50.06°, 53.56°, 56.45°, and 57.37°, while the XRD peak at 33.98° is assigned to Cu₂O, with the existence of the related peak of amorphous carbon at 22.97° and graphite oxide at 14.45°.^43,44 The diffractogram of SA beads illustrates the semi-crystalline XRD pattern, where there are amorphous wide peaks at 2-theta of at 13.58° and 21.66°, with emerging of lower intense crystalline peaks at 30.45°, 31.70°, 34.46°, 40.73° and 45.43°. The crystallographic pattern of SDS-MIL88A/Cu-BC@SA beads denoted that most of diffraction peaks of SDS-MIL88A and Cu-BC shielded after incorporating into the polymeric SA shell.

3.1.3 Zeta potential. The removal of amphoteric contaminants is a big challenge since the coulombic repulsion represents an opposing effect that handicaps reaching the targeted pollutants to the catalyst. The Tc drug naturally exists in three forms based on the pH medium: cationic (pH < 3.3), anionic (pH > 7.7), and zwitterionic (3.3 < pH < 7.7). Pioneering studies have reported the favorability of applying a catalyst with zwitter character in removing the amphoteric Tc at pHs between 3.3 and 7.7 to conquer the negative impact of the coulombic repulsion. In this case, a coulombic attraction force could be generated between the zwitter Tc and the zwitter catalyst. Consequently, the zeta potential values of SDS-MIL88A/Cu-BC@SA beads were investigated at various pH media to pick up their point of zero charge, where the beads are in the zwitter state. Fig. 1c demonstrates that the zero charge point of SDS-MIL88A/Cu-BC@SA beads at pH = 6.68, suggesting the favorability of near alkaline media to remove Tc by the beads.⁴⁵ Additionally, the zeta potential curve elucidated the superb capability of the SDS-MIL88A/Cu-BC@SA to remove anionic pollutants in acidic media and cationic pollutants at alkaline conditions, reflecting the versatility of the fabricated beads.

3.1.4 SEM. Fig. 2 elucidates the SEM images of MIL88A, SDS-MIL88A, BC, Cu-BC, SA, and SDS-MIL88A/Cu-BC@SA beads. The SEM of MIL88A clarifies the distinctive rod shape (Fig. 2a), which is the typical outer shape of the synthesized MIL88A by the hydrothermal approach. For the SEM of SDS-MIL88A (Fig. 2b), there are assembled particles on the MIL88A surface, which belong to the SDS particles and confirm the successful SDS-functionality on the MIL88A. The outer surface of the beads shows a spherical morphology with a rough, rock-like texture (Fig. 2c), forming wide holes that enable it to act as a remarkable supporter. For the SEM of Cu-BC, the sheets and holes of BC are almost totally covered by the spheroidal copper particles, assuring the occurrence of Cu-doping over the BC surface, as shown in Fig. 2d. The SEM image of the pristine SA beads showed a spherical-shaped morphology with a rough, porous surface, as shown in Fig. 2e and f. The core of the SA beads (Fig. 2g and h) contains channel-like pores, which are available to hold the incorporated SDS-MIL88A and Cu-BC substances. For the SEM of the SDS-MIL88A/Cu-BC@SA beads (Fig. 2i and j), the shape of the outer shell reveals spheres with a rocky texture, implying the role of the incorporated SDS-MIL88A and Cu-BC in improving the mechanical strength of SA beads. In addition, the core of the SDS-MIL88A/Cu-BC@SA beads demonstrated the presence of SDS-MIL88A and Cu-BC inside the SA shell, forming a core–shell structure, as shown in Fig. 2k and l.

Fig. 2 SEM images of (a) MIL88A, (b) SDS-MIL88A, (c) BC, (d) Cu-BC, (e and f) outer-surface of SA beads, (g and h) core of SA beads, (i and j) outer-surface of SDS-MIL88A/Cu-BC@SA beads, and (k and l) core of SDS-MIL88A/Cu-BC@SA beads.

3.1.5 XPS. Fig. 3a–f demonstrates the XPS results of the elemental compositions of the SDS-MIL88A/Cu-BC@SA. The wide spectrum of SDS-MIL88A/Cu-BC@SA reveals the presence of oxygen, carbon, iron, sulfur, and copper in the matrix of the beads, with atomic percentages of 40.57, 41.13, 12.31, 2.03, and 3.96%, respectively. The Cu2p spectrum demonstrated the characteristic XPS peaks of monovalent copper at 935.54 and 951.83 eV, which are ascribed to Cu2p_3/2 and 2p_1/2 with an atomic % of 40.04%. In addition, the divalent copper peaks of 2p_3/2 and 2p_1/2 manifested at 939.19 and 955.42 eV and their total atomic % was 28.32%.⁴⁶ The Fe2p spectrum shows the related peaks to ferrous of 2p_3/2 and 2p_1/2 at 711.83 and 723.55 eV with atomic % of 44.10%, while the XPS peaks at 715.35 and 727.17 eV, which are accompanied by ferric of 2p_3/2 and 2p_1/2 (23.40%).⁴⁷ The C1s spectrum reveals the carbon functional groups: C–C, COO, and C–O at 284.79, 288.55, and 285.58 eV.⁴⁸ The S2p spectrum confirmed the SDS-functionality on the MIL88A surface as the corresponding peaks to SO₄²⁻ and S–O of SDS were observed at 169.45 and 166.57 eV, respectively.⁴⁹ For the O1s spectrum, three XPS peaks appeared at 531.98, 533.17, and 535.79 eV, which are associated with Cu/Fe–O, COO, and C–O.

Fig. 3 The XPS results of the fresh SDS-MIL88A/Cu-BC@SA catalyst: (a) spectrum survey, (b) Cu2p, (c) Fe2p, (d) C1s, (e) S2p, and (f) O1s.

3.2 Optimization of the Tc Fenton-like reaction

3.2.1 Comparison study. The catalytic activities of MIL88A, SDS-MIL88A, BC, Cu-BC, neat SA beads, and SDS-MIL88A/Cu-BC@SA beads toward Tc were compared in an adsorption/Fenton-like mode, as shown in Fig. 4a. The adsorption % of MIL88A and SDS-MIL88A were 28.61% and 32.88%, and their degradation % were 76.28% and 86.12%, reflecting the impact of surfactant functionality on the activity of MIL-88A. This enhancement could be explained by the presence of sulfate groups in SDS, which could improve adsorption capacity by interacting with Tc via n-pi interaction. Additionally, SDS contributes to the recovery of Fe²⁺ by increasing the reduction of Fe³⁺, which increases the concentration of Fe²⁺ on the SDS-MIL88A surface.⁴⁷

Fig. 4 The results of the lab experiments of the degradation of Tc by SDS-MIL88A/Cu-BC@SA in adsorption/Fenton-like manner: (a) comparison test between SDS-MIL88A/Cu-BC@SA and its authentic components toward degrading Tc, (b) investigating the best pH, (c) identifying the ideal SDS-MIL88A/Cu-BC@SA dose, (d) determining the optimal H₂O₂ concentration, (e) studying the thermal behavior of the catalytic Tc/SDS-MIL88A/Cu-BC@SA system, and (f) investigating the catalytic capability of SDS-MIL88A/Cu-BC@SA toward different Tc concentrations.

Moreover, the comparison experiments clarified an amelioration in the catalytic activity of BC after Cu-doping, where the adsorption aptitude of Tc onto BC and Cu-BC recorded 20.06% and 27.17%, and the degradation efficacy were 61.73% and 74.26%. Cu-doping improved the adsorption of Tc since the Cu species and Tc could bond by forming coordination bonds and Lewis acid–base interaction. Additionally, the Cu ions could share electrons to activate H₂O₂ and produce ˙OH, boosting the Fenton-like degradation efficiency of the Tc molecules.⁴⁸

The adsorption efficiencies of neat SA beads and SDS-MIL88A/Cu-BC@SA beads toward Tc were 15.97% and 44.91%, while the degradation % were 45.84% and 95.12%. These results demonstrated that SA is not just a template to carry SDS-MIL88A/Cu-BC, but it possesses good catalytic activity. Furthermore, SA provides a fast separation of the SDS-MIL88A/Cu-BC@SA beads from the catalytic medium by a colander, which improves their recycling and reusing for degrading Tc for sequential catalytic runs with an excellent catalytic activity.⁵⁰

3.2.2 Impact of pH medium. Fig. 4b depicts the catalytic activity of the SDS-MIL88A/Cu-BC@SA beads toward degrading the Tc molecules at different catalytic pH media. The recorded results revealed a boost in the adsorption% of Tc from 28.50% to 44.91% and the degradation% from 76.54% to 95.12% by escalating the pH from 3 to 7. This result demonstrates governing the coulombic attraction of the zwitter Tc and the zwitter SDS-MIL88A/Cu-BC@SA beads and overcoming the coulombic repulsion effect on decreasing the adsorption aptitude of the amphoteric Tc drug. Meanwhile, the redox potential values of H₂O₂ in acidic and alkaline media are 2.7 and 1.8 V, meaning that the production amounts of ˙OH in alkaline or near alkaline media are higher than those in the acidic media.⁵¹ In addition, the high concentrations of H⁺ in the acidic media that retard the activity of ˙OH in the catalytic Tc/SDS-MIL88A/Cu-BC@SA system.

Conversely, raising the pH over 7 caused a slight diminishment in the adsorption% of Tc and the degradation% to 42.21% and 91.33%, while at pH = 11, they sharply declined to 40.47% and 87.79%. This result may be assigned to producing hydro-peroxy species (OOH⁻) because of the excess hydroxyl in the alkaline media that reacts to H₂O₂, as clarified in eqn (2). The OOH⁻ has a higher activity toward the metal ions than H₂O₂, so it can interact with Fe and prevent it from sharing electrons for activating H₂O₂, as elucidated in eqn (3).⁵² Additionally, H₂O₂ exhibits a limitation in highly alkaline media, which is the auto-decomposition (eqn (4)).

H₂O₂ + OH⁻ → OOH⁻ + H₂O (2)

Fe + OOH⁻ → Fe…˙OOH (3)

2H₂O₂ → O₂ + 2H₂O (4)

3.2.3 Impact of the catalyst dose. The appropriate SDS-MIL88A/Cu-BC@SA dose was optimized by evaluating the catalytic degradation efficacy of Tc using different amounts of the beads, as revealed in Fig. 4c. The Tc adsorption% increased from 34.88 to 40.26, 44.91, and 55.34% by augmenting the weight of SDS-MIL88A/Cu-BC@SA from 7 to 15, 20, and 25 mg because of increasing the available functional groups for interacting with the Tc molecules and adsorbing them onto the surface of the beads. Likewise, such an increment in the SDS-MIL88A/Cu-BC@SA dose enhanced the Fenton-like degradation% of Tc from 76.85 to 87.45, 95.12, and 100%. This result can be demonstrated by enriching the active groups in the catalytic medium that could activate H₂O₂ and yield ˙OH radicals.⁵³

3.2.4 Impact of oxidant concentration. Fig. 4d illustrates the efficacy of the Fenton-like degradation of the Tc molecules by the SDS-MIL88A/Cu-BC@SA beads in the presence of different H₂O₂ concentrations. Raising the concentrations of H₂O₂ from 10 to 100 mg L⁻¹ strengthened the degradation aptitude of SDS-MIL88A/Cu-BC@SA toward Tc, where it escalated from 78.70% to 95.12%. This catalytic performance may be due to the magnification of the production of ˙OH when the H₂O₂ concentration was raised inside the Fenton-like catalytic system, which directly boosted the concentration of the degraded Tc molecules. However, over-escalating the concentration of H₂O₂ to 200 mg L⁻¹ had the opposite results, leading to a decrease in the degradation% of Tc to 92.01%, where the superfluity of the H₂O₂ molecules in the catalytic medium can attack the generated ˙OH, as depicted in eqn (5) and (6).

H₂O₂ + ˙OH → HOO˙ + H₂O (5)

HOO˙ + ˙OH → H₂O + O₂ (6)

3.2.5 Impact of the reaction temperature. The Fenton-like reaction of degrading Tc by SDS-MIL88A/Cu-BC@SA was studied at varied reaction temperatures, as elucidated in Fig. 4e. The adsorption% of Tc onto the SDS-MIL88A/Cu-BC@SA beads declined from 44.91% to 30.18% because of fastening the Brownian motion of the Tc molecules, increasing the attained molecules to be on the surface of the beads. Similarly, the Fenton-like degradation% diminished from 95.12% to 72.50%, which is most likely due to the higher activity of ˙OH, resulting in its self-consumption, as represented in eqn (7).⁵⁴ This finding reflected that room temperature is the favorable reaction temperature to degrade Tc, clarifying the potentiality of the catalytic Tc/SDS-MIL88A/Cu-BC@SA system that saves energy.

˙OH + ˙OH → H₂O₂ (7)

3.2.6 Impact of the Tc concentrations. Fig. 4f shows the Fenton-like degradation capability of the SDS-MIL88A/Cu-BC@SA beads toward the escalated concentrations of the Tc molecules. The experimental findings revealed an attenuation in the adsorption% of Tc from 44.91% to 17.36%, after elevating its concentrations from 50 to 300 mg L⁻¹. Also, after such augmentation in the Tc concentrations, the degradation% of Tc decreased from 100% to 70%. These results can be anticipated by the lower active site quantities compared to the high concentrations of Tc, rendering the available sites insufficient to adsorb the high concentrations of Tc. Furthermore, the created concentration of the ˙OH is not enough to attack and degrade Tc with high concentrations.⁵³

3.3 Kinetic investigations

First-order and Second-order expressions (eqn (8) and (9)) were applied to analyze the experimental data of the Fenton-like degradation reaction of Tc by the SDS-MIL88A/Cu-BC@SA beads. The plots of First-order and Second-order of the Tc degradation process by SDS-MIL88A/Cu-BC@SA (Fig. 5a and b) demonstrated the favorability of Second-order to model the reaction, in which the correlation coefficients of the Second-order curves are larger than those of First-order (Table 1). Moreover, the calculated rate constants of the degradation reaction of Tc by the SDS-MIL88A/Cu-BC@SA beads under the Second-order model were 0.2721, 0.0605, 0.0235, and 0.0162 L mol⁻¹ min⁻¹ for the Tc concentrations of 50, 100, 200, and 300 mg L⁻¹, respectively. The rate constant for the degradation process of Tc molecules decreased as their concentrations were increased, which is most likely due to the complex balancing act of many parameters in the catalytic system, like the competition possibility between Tc and the degraded compounds, as well as the production and consumption of ˙OH.^55–57k₁ and k₂ symbolize the rate constants of the First-order and Second-order models.

Fig. 5 Kinetic assessments of the Fenton-like degradation Tc by the SDS-MIL88A/Cu-BC@SA beads: (a) first-order and (b) second-order, and (c) quenching test of the Fenton-like Tc degradation by the SDS-MIL88A/Cu-BC@SA beads.

Table 1 Kinetic parameters of the degradation reaction of Tc by the SDS-MIL88A/Cu-BC@SA beads

Kinetic model	Tc concentrations (mg L^−1)
	50	100	200	300
First order
k1	0.0379	0.0135	0.0086	0.0077
R^2	0.962	0.859	0.854	0.874
Second order
k2	0.2721	0.0605	0.0235	0.0162
R^2	0.985	0.965	0.952	0.948

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3.4 Quenching study

Generally, the hydroxyl and superoxide (O₂˙⁻) are the governing ROS in the Fenton and Fenton-like reactions. So, it is essential to scrutinize which radical between O₂˙⁻ and ˙OH controlled the Fenton-like degradation process of Tc by the SDS-MIL88A/Cu-BC@SA beads. t-BuOH and TCM were selected as the quencher for ˙OH and O₂˙⁻ owing to their higher activity, where the k values of BuOH and TCM are 5.2 × 10⁸ and 3 × 10¹⁰ M⁻¹ s^−1.58 Fig. 5c depicts the results of the quenching study of the degradation process of Tc by the SDS-MIL88A/Cu-BC@SA beads. The recorded result from the quenching study demonstrated that ˙OH is the controlling ROS in the Fenton-like process of Tc by SDS-MIL88A/Cu-BC@SA, since the degradation% of Tc were 89.82% and 65.85% in the presence of TCM and BuOH, while the degradation% in the absence of scavenger was 95.12%.⁵⁹ Additionally, the quenching study indicated the occurrence of the degradation process of Tc by SDS-MIL88A/Cu-BC@SA via a radical mechanism, not the oxygen vacancies.^60–62

3.5 Recycling/reusing investigation

Fig. 6a signalized the recycling/reusing study of the SDS-MIL88A/Cu-BC@SA beads for five cycles of the adsorption/Fenton-like degradation of Tc. The results demonstrated that the adsorption % of Tc declined from 44.91 to 36.48%, and the degradation% diminished from 95.12 to 85.54%. This decrease in the degradation efficiency of the SDS-MIL88A/Cu-BC@SA beads after being used for five sequential Tc degradation cycles because of the potent chemical interactions between Tc and the beads, leading to a block of some active catalytic groups. Additionally, some active catalytic groups of the SDS-MIL88A/Cu-BC@SA beads may be depleted during the activation of H₂O₂, as the redox reaction does not fully regenerate all the oxidized metal species. Fig. 6b demonstrates that there is no change in the main diffraction peaks of SDS-MIL88A/Cu-BC@SA beads after the adsorption/Fenton-like degradation of Tc, confirming their structural stability. A slight decrease in peak intensity is observed, which can be attributed to partial blockage of functional groups by adsorbed Tc molecules, rather than to any collapse of the catalyst structure.

Fig. 6 (a) Recycling study of the SDS-MIL88A/Cu-BC@SA beads toward the adsorption/Fenton-like degradation of Tc during five cycles, (b) XRD of the SDS-MIL88A/Cu-BC@SA beads before and after the Tc degradation reaction, and (c) desorption profile of Tc after Fenton-like degradation.

3.6 Adsorption-assisted Fenton degradation of Tc at the bead–solution interface

In bead-assisted heterogeneous Fenton systems, adsorption plays an active role in the degradation process rather than acting as a separate removal pathway. In this study, Tc adsorption reached 44.91%, indicating effective accumulation of the pollutant at the bead surface prior to oxidation. After introducing the Fenton reagent, the overall removal increased to 95.12%, demonstrating that the adsorbed Tc was readily involved in the degradation process. Desorption experiments (Fig. 6c) further revealed that only 8.45% of the initial Tc concentration (corresponding to 36.6% of the adsorbed fraction) could be recovered, indicating that the majority of surface-associated Tc was transformed during the reaction. Collectively, these observations highlight the role of adsorption in lowering the effective concentration barrier and facilitating efficient surface-driven Fenton degradation. The role of adsorption in the present system is not limited to passive uptake of Tc from solution. Instead, adsorption enriches Tc molecules near the catalytic surface and reduces the diffusion barrier between the pollutant and the reactive sites. This interfacial enrichment increases the local substrate concentration around Fe-, Cu-, and EPFR-containing domains, which facilitates immediate oxidation of the surface-associated Tc molecules once H₂O₂ is activated. Therefore, adsorption and Fenton-like oxidation act cooperatively at the bead–solution interface rather than as two independent sequential processes. This interpretation is also consistent with the observed high overall removal efficiency and the second-order kinetic fitting, which suggest a surface-mediated adsorption-coupled degradation pathway.

3.7 Mechanism of Tc removal

The XPS survey spectrum of the SDS-MIL88A/Cu-BC@SA affirmed the occurrence of the adsorption of Tc, where the corresponding peak of nitrogen manifested at 400.37 eV with an atomic% of 0.49% (Fig. 7a). The Tc molecules may adsorb onto the SDS-MIL88A/Cu-BC@SA surface by many pathways, as follows. The coordination bonds: SDS-MIL88A/Cu-BC@SA beads contain unsaturated metals, which are copper and iron that can bond to the hydroxyl and amine groups of Tc via coordination bonds.^63–65 In addition, the hydrogen bonding, the presence of nitrogen and oxygen in the Tc structure, facilitates its attraction from the bulk solution onto the surface of SDS-MIL88A/Cu-BC@SA by forming H-bonding between those species of Tc and the hydrogen atoms of the composite. Meanwhile, the oxygen atoms of SDS-MIL88A/Cu-BC@SA can construct H-bonding with the hydrogens of the Tc molecules.^64,66

Fig. 7 XPS spectra of the used SDS-MIL88A/Cu-BC@SA catalyst in degrading Tc: (a) spectrum survey, (b) Cu2p, (c) Fe2p, (d) O1s, and (e) S2p.

The pi–pi interaction: aromatic rings in the structures of Tc and SDS-MIL88A/Cu-BC@SA, enable the adsorption of Tc onto the beads by the formation of pi–pi interactions. Meanwhile, the presence of electron-donating groups in SDS-MIL88A/Cu-BC@SA, like the sulfate and hydroxyl species, can share electrons to fill the empty pi-orbital of the Tc molecules throughout the n-pi interaction.⁶⁷ Furthermore, the Lewis acid/base interaction: the Tc molecules involve electron donor species (Lewis base) such as hydroxyl and amine, while the SDS-MIL88A/Cu-BC@SA beads have electron acceptor species (Lewis acid) like copper and iron. Therefore, the Lewis acid/base interaction is a possible adsorption pathway in the Tc adsorption reaction onto the SDS-MIL88A/Cu-BC@SA beads.

The XPS spectra of the used SDS-MIL88A/Cu-BC@SA beads of copper, iron, oxygen, and sulfur showed changes in the peak positions, reflecting their participation in the activation of H₂O₂ molecules for creating ˙OH radicals. The copper species of SDS-MIL88A/Cu-BC@SA can activate H₂O₂, as elucidated in eqn (10), and yield the ˙OH radicals that attack the Tc molecules and degrade them to less or non-toxic compounds. Fig. 7b represents the XPS spectrum of the copper species of the used SDS-MIL88A/Cu-BC@SA beads. The mono-valent copper peaks at 935.54 and 951.83 eV, shifted to 935.48 and 952.62 eV, respectively. Meanwhile, the related peaks to the divalent copper shifted from 939.19 and 955.42 eV to 940.79 and 955.37 eV sequentially. The ratios of mono/di-valent copper in pure and used were 1.41 and 1.32, clarifying a decline in the atomic% of the mono-valent copper and an increase in that of di-valent copper after the Fenton-like degradation of the Tc molecules. These findings assured the contribution of the copper species of SDS-MIL88A/Cu-BC@SA to the degradation of the Tc molecules. The copper species not only generate ˙OH, but also recover ferric ions to ferrous, as represented in eqn (11), where the redox potential values of copper and iron are 0.15 and 0.77 V, respectively.

Cu⁺ + H₂O₂ → Cu²⁺ + ˙OH + ⁻OH (Cu⁺/Cu²⁺ = 0.15 V) (10)

Cu⁺ + Fe³⁺→ Cu²⁺ + Fe²⁺ (11)

The iron spectrum of the SDS-MIL88A/Cu-BC@SA beads after the Tc degradation reaction (Fig. 7c) signals a shift in the ferric and ferrous species, indicating the role of iron inside the catalytic Tc/SDS-MIL88A/Cu-BC@SA system. The peaks of the ferrous ions shifted from 711.83 and 723.55 eV to 710.97 and 724.08 eV; in addition, the position of ferric peaks changed from 715.35 and 727.17 eV to 713.53 and 727.02 eV. Eqn (12) represents how ferrous of SDS-MIL88A/Cu-BC@SA could activate H₂O₂ to create the active ˙OH radicals. The decline in the ferrous/ferric ratios from 1.89 to 1.49 eV after the degradation process of Tc, reflecting the consumption of a part of the ferrous species on activating the H₂O₂ molecules.

Fe²⁺ + H₂O₂ → Fe³⁺ + ˙OH + ⁻OH (Fe²⁺/Fe³⁺ = 0.77 V) (12)

The presence of BC in SDS-MIL88A/Cu-BC@SA plays a double role in the Fenton-like catalytic cycle, where it contains EPFRs that can activate H₂O₂ for creating the ˙OH radicals and proceeding the initial half of the catalytic cycle, as elucidated in eqn (13). Furthermore, BC contributes to the other half of the catalytic cycle, as it can regenerate the oxidized metal species to continue the redox cycle, recovering the monovalent copper and ferrous ions. The O1s spectrum of SDS-MIL88A/Cu-BC@SA after the Fenton-like reaction of Tc showed changes in the positions of Cu/Fe–O and COO peaks from 531.98 and 533.17 eV to 530.79 and 532.00 eV (Fig. 7d). Meanwhile, pioneering investigations into the role of SDS in the Fenton-like catalytic cycle have revealed that SDS accelerates the reduction of metal ions, thereby regenerating them and prolonging the redox cycle. This enhancement increases the production of ˙OH radicals and directly promotes the decomposition rate of Tc molecules. The XPS spectrum of the sulfur species of the used SDS-MIL88A/Cu-BC@SA beads demonstrated shifting of SO₄²⁻ and S–O peaks from 169.45 and 166.57 eV to 168.54 and 167.86 eV, as illustrated in Fig. 7e, indicating their participation in the Fenton-like decomposition of the Tc molecules.

EPFRs + H₂O₂ → ˙OH + ⁻OH (13)

The above-mentioned catalytic reactions can activate H₂O₂ molecules, producing ˙OH that attacks Tc molecules, causing their decomposition via consequent reactions, which yield varied intermediates and finally carbon dioxide and water, as represented in eqn (14).

Tc + ˙OH → intermediates → CO₂ + H₂O (14)

TOC analysis further supported the proposed degradation pathway. Under the optimized adsorption-coupled Fenton-like conditions, the TOC removal reached approximately 65.9%, indicating that a considerable fraction of the organic carbon in Tc was mineralized during the reaction. Importantly, the TOC removal was lower than the total Tc decomposition efficiency, suggesting that part of Tc was converted into smaller organic intermediates rather than being fully converted to CO₂ and H₂O. This result is consistent with the GC-MS analysis and confirms that Tc removal by SDS-MIL88A/Cu-BC@SA beads involved the decomposition of the parent Tc molecules into smaller intermediates, together with partial mineralization through ˙OH-driven oxidation. A schematic representation of the possible adsorption/Fenton-like mechanisms for the removal of Tc by SDS-MIL88A/Cu-BC@SA beads is shown in Fig. 8.

Fig. 8 A schematic representation for the possible adsorption/Fenton-like mechanisms for the removal of Tc by SDS-MIL88A/Cu-BC@SA beads.

3.8 Identifying the decomposition intermediate of Tc

The degradation intermediates of Tc were identified by GC-MS analysis, and the possible degradation pathway is proposed in Fig. 10. The decomposition process can be divided into four main steps. In the first step, Tc loses two H₂O molecules to form the corresponding aromatic intermediate (I). This intermediate then undergoes ˙OH-induced oxidative cleavage, producing the key fragments II and III. In the second step, fragment II was further degraded through oxidative cleavage, producing oxalic acid (V) and acetylsalicylic acid (IV). Acetylsalicylic acid was then decarboxylated to form m-hydroxyacetophenone (VI). The formation of oxalic acid was supported by the GC-MS result, where the detected m/z value of 91.1 corresponded to [M + 2H]. Oxalic acid can be further decomposed into CO₂ and H₂O during the oxidation process. In the third step, the reactive aldehyde intermediate III was oxidized to the corresponding carboxylic acid intermediate VII. The detected m/z value of 284.1 was consistent with the calculated m/z value of 284.06. Intermediate VII then underwent dimethylamine removal in the presence of H₂O₂ to form intermediate VIII. Further oxidation of VIII produced the unstable carbamic acid intermediate IX and oxaloacetic acid X. Carbamic acid can rapidly decompose into NH₃ and CO₂, while oxaloacetic acid showed a short retention time of less than 3.0 min due to its limited stability. The detected m/z value of 133.4 was assigned to [M + H] of oxaloacetic acid. In the fourth step, decarboxylation of oxaloacetic acid X produced malonic acid XI, which was further oxidized to glycolic acid XII. The detected m/z value of 77.1 was consistent with the [M + H] signal of glycolic acid. Overall, the GC-MS results indicate that Tc degradation proceeded through dehydration, oxidative cleavage, decarboxylation, demethylation, and further oxidation reactions. These reactions produced smaller organic intermediates, some of which could be further decomposed to CO₂, H₂O, and NH₃. This degradation pathway is consistent with the TOC results, which confirmed partial mineralization rather than complete conversion of all organic carbon into inorganic products within the reaction time (Fig. 9).

Fig. 9 GC-MS analysis of the degraded Tc by SDS-MIL88A/Cu-BC@SA beads.

Fig. 10 The possible intermediate compounds that are formed during the Fenton-like decomposition reaction of the Tc drug by SDS-MIL88A/Cu-BC@SA beads.

3.9 Comparison study

As summarized in Table 2, several reported Tc degradation systems show high efficiency, but many require photo-assistance, electro-Fenton conditions, ultrasound, strongly optimized acidic conditions, or substantially longer reaction times. In contrast, the present SDS-MIL88A/Cu-BC@SA beads system achieved 95.12% tetracycline removal within 60 min at pH 7, while also integrating adsorption enrichment and catalyst recoverability in a single bead microreactor platform.

Table 2 Comparison between the Tc removal efficiency by SDS-MIL88A/Cu-BC@SA beads and previous reported catalysts

Catalyst	Process	pH	Time (min)	R (%)	Ref.
Fe3O4	Ultrasound-assisted Fenton-like	7	60	93.6	68
MnFe2O4/diatomite	Visible photo-Fenton	7	60	91.8	69
(Ce0·33Fe)MIL-88A/10% g-C3N4	Fenton-like	7	100	92.44	70
CoFe-LDH/MoS2	Fenton-like	3–11	60	>85	71
EMMCS-G	Fenton-like	3	360	95.6	72
30% ZnWO4/ZnFe2O4	Photo-Fenton	5	60	92.1	73
Immobilized FeWO4	Electro-Fenton	7	60	100	74
MIL-101(Fe)/g-C3N4/FeOCl	Photo-Fenton	3–7	60	>94	75
SDS-MIL88A/Cu-BC@SA beads	Adsorption-coupled Fenton-like	7	60	95.12	This study

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4 Conclusion

In summary, SDS-MIL88A/Cu-BC@SA beads were successfully fabricated, and each preparation step was confirmed using different characterization techniques. FTIR, XRD, and SEM analyses confirmed the formation of SDS-functionalized MIL-88A and Cu-doped BC, as well as their successful incorporation into the SA bead matrix. In addition, the ZP plot implied that the zero-charge of the SDS-MIL88A/Cu-BC@SA surface was at pH = 6.68. The catalytic experiments indicated the effective role of the adsorption-coupled Fenton-like process, since Tc adsorption reached 44.91% and the decomposition efficiency reached 95.12% within 60 min at room temperature, neutral pH, Tc concentration of 50 mg L⁻¹, H₂O₂ concentration of 100 mg L⁻¹, and bead dosage of 20 mg. Moreover, TOC analysis showed that 65.9% TOC removal, confirming the partial mineralization of Tc molecules, with some of the parent molecules converted into smaller organic intermediates. The kinetic study of the Tc Fenton-like decomposition by the SDS-MIL88A/Cu-BC@SA beads was best represented by the Second-order model with a higher decomposition rate constant of 0.2721 L mol⁻¹ min⁻¹. The scavenging study demonstrated that the dominant role of ˙OH radicals on the Fenton-like decomposition of Tc by SDS-MIL88A/Cu-BC@SA. The reusability study confirmed the viability of molding highly dispersive catalysts into a bead shape, which provides easy separation of the catalyst from the reaction media without loss of weight. The mechanistic study is supposed to occur through the adsorption reaction of Tc by coordination bonds, Hydrogen Bonding, n-pi interactions, pi–pi interactions, and Lewis acid–base interactions. In addition, the Fenton-like decomposition of Tc proceeded by activating H₂O₂ using ferrous, monovalent copper ions, and EPFRs, and SDS and EPFRs expanded the redox cycle by recovering the active metal species.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra02195f.

Funding

This work was supported by “the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia [Project No. KFU262748]”.

Acknowledgements

The authors acknowledge the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research at King Faisal University, Saudi Arabia, for financial support under the annual funding track [KFU262748].

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